Since the introduction of type-only imports in TypeScript 3.8, a lot of developers including me have been confused about the usage of type-only imports. If we read the official docs, it says -

import type only imports declarations to be used for type annotations and declarations. It always gets fully erased, so there’s no remnant of it at runtime.

Similarly, export type only provides an export that can be used for type contexts, and is also erased from TypeScript’s output.

But isn't typescript code supposed to cease automatically at runtime? Why do we need type-only imports?

To understand this, First, we need to understand how single-file transpilers (like Babel) work.

Suppose we have the following 3 files -

1. Component.tsx

    import { Props } from "./types";
   
    const Component = (props: Props) => {
      return <div>Component</div>;
    };
   
    export { Component };
   

2. types.ts

    export type Props = {
      a: number;
      b: string;
    };
   

3. index.ts

    import { Component } from "./Component";
    import { Props } from "./types";
   
    export { Component, Props };
   

Now, let's see what will the corresponding output JS files look like when they are passed through a single file transpiler -

1. Component.js

    "use strict";
    exports.__esModule = true;
    exports.Component = void 0;
    var Component = function (props) {
        return <div>Component</div>;
    };
    exports.Component = Component;
   

2. types.js

    "use strict";
    exports.__esModule = true;
   

3. index.js

    "use strict";
    exports.__esModule = true;
    exports.Props = exports.Component = void 0;
    var Component_1 = require("./Component");
    exports.Component = Component_1.Component;
    var types_1 = require("./types");
    exports.Props = types_1.Props;
   

If we compare the above outputs, we can see that all the typescript-only code (types and imports) has been successfully removed in Component.tsx and types.ts file but not in index.js file.

If we run this JS code, we will get a runtime error ❗️ since there is no named export Props in types.js file

Now, let's see how our transpiler was able to remove typescript-only code in each of the files -

1. Component.tsx - Here, the import Props is being used as a type in the file itself, So it is able to deduce that Props is indeed a type and can safely remove it.

    import { Props } from "./types";
                              👇
    const Component = (props: Props) => {
   

2. types.ts - In this file, we have used the type keyword to declare Props type. So, it can also be safely removed.

           👇
    export type Props = {
   

3. index.ts - In this file, we are simply importing and re-exporting Component and Props. Since single-file transpilers work only on a single file at a time, there is no way to know whether these are types or variables or functions, etc.

   So, they both are kept in the output js file.

    import { Component } from "./Component";
    import { Props } from "./types";
   
           ❔❔❔
    export { Component, Props };
   

To fix this, we can mark Props import as type-only -

import { Component } from "./Component";
import type { Props } from "./types";

export { Component, Props };

Now, our transpiler knows for sure that Props is a type and can safely omit it.

Note: This problem only occurs when we are using a single file transpiler such as babel since they only operate on a single file at a time and can't apply code transforms that depend on understanding the full type system.

TypeScript also provides us with a flag isolatedModules to avoid writing certain code (like the above) that can cause issues when using a single-file transpiler.

If we turn on the isolatedModules flag, we will get the following error in index.ts file

import { Component } from "./Component";
import { Props } from "./types";

 // ❗️❗️ Re-exporting a type when the '--isolatedModules' flag is provided requires using 'export type'. ❗️❗️
export { Component, Props };

So, to summarize this 🔖-

• If we are importing a type and using it in the same file, we do not need to use type since even a single file transpiler can understand from the context that it is a type .

• But, If we are only exporting the type and not using it, we need to make it a type-only import to explicitly tell the transpiler that it is a type.

• We should turn on the isolatedModules flag to avoid such issues.